1. Field of the Invention
This invention relates to archival memories and particularly to electron or ion beam accessible memories for storing extremely large data bases. More particularly, the present invention relates to methods and devices for data storage in archival, beam accessible memories.
2. Brief Description of the Background Art
Fixed head magnetic media devices used for data storage in modern computer systems are limited both by slow access times and the large physical data storage media. One consequence is that on-line data bases larger than about 10.sup.12 bits are not feasible. Beam accessible memories have the potential for solving these problems. Both electron and ion beams are attractive because of their ease of deflection, small spot sizes and appreciable energy density.
The pioneering work of Wolfe, Bennett and Ledges in 1969 demonstrated that 100 nm. recording at 10 Mbit/second data rates by electron beam micromachining of arsenic/selenium glasses at power levels of 10.sup.7 W/cm..sup.2 and 10 kV accelerating voltage were possible. They subsequently showed that the bit size could be reduced to 70 nm. by using a gold loaded target and 4 kV beam voltage to improve localization of the absorbed power. Information was stored by selectively evaporating pits in the film. Secondary emission contrast was used for information retrieval. The main drawback of this approach resulted from the unpredictability of secondary emission contrast for readout. Since secondary emission yield is sensitive to surface contamination and charge on the memory target, the archival lifetime of the data was short and unpredictable.
In U.S. Pat. No. 4,081,794, Kirkpatrick and Parks disclosed a storage scheme where junction microdiodes were formed by electron beam alloying. They subsequently showed that the power densities required to achieve high resolution alloying on these highly conductive substrates cannot be obtained with state of the art electron optics. The high thermal conductivity of the silicon substrate of the diode made it impossible to localize the beam energy.
A method for writing on an archival memory target wherein a masking layer is melted, is disclosed in U.S. Pat. No. 4,287,572. The melted material "balls up" when it solidifies to create a transmission "window" for a scanning electron beam.
Although archival electron beam addressed memories have been the subject of active research for over fifteen years, there is still an unfulfilled demand for a memory system that possesses both good contrast and high thermal isolation between bit memory locations.